Xis protein primarily promotes
Directionality of lambda recombination controlled by accessory proteins dictating which attachment sites synapse. Integration intasome formed by Int plus IHF favors attP x attB recombination generating attL and attR flanking prophage. For prophage escape after induction, Xis protein synthesized binds X binding sites in attR introducing bends that prevent Int from occupying P-type sites while favoring excisive contacts. Xis also contacts Fis and Int N-terminal domain redirecting tetramer towards attL x attR excisive mode. Thus Xis shifts equilibrium from integrative to excisive state promoting
Ref: Watson et al., Molecular Biology of the Gene, 7th ed., Chapter 12: Xis Promotion of Lambda Excision
